The moon audio TRRS plug, or any TRRS plug is what is needed to convert from TRS to TRRS.
You cannot use a plug or cable converter to convert TRS to TRRS. The TRS has the L & R grds soldered together, so there is no way to split it out. Going the other way is possible, as you can take the TRRS with separate grds and bridge them together to TRS. If you can find an adaptor plug or cable to do that, or make one.
To get TRRS to an IEM, you have to find a cable wired for TRRS audio, like the ones made for HiFiMan 900, with the proper termination at the other end to match your IEM's plug. It has to be a cable made for a TRRS audio output, and not a TRRS cable made to add a microphone. Most of the TRRS cables available use the TRRS contacts as a standard TRS, with the 4th contact used for the microphone. (the 4 TRRS contacts are wired as TRS & M(microphone)) So the L- and R- grds are still bridged together, just like in a normal TRS cable.
Or, remove the TRS plug from a cable that fits your IEM, and solder in a TRRS plug, splitting the L- and R- onto the R and S contacts on the plug. This is what I did with my Zephone cable for the TF10. Unsoldered the TRS plug, cut back on the cable insulation about 3/4" to expose more of the wire for soldering.
Bear in mind that soldering the small flat surfaces on a TRRS plug is finicky. I found that stripping a longer length of wire, wrapping it around the plug contact surface, soldering it quickly and lightly (don't overheat it), and cutting off the excess wire was the only way. Use a low temperature solder iron with a fine tip. Or solder very quickly with a hotter soldering iron.
Solder - I use WBT solder with a small percentage of silver, and a low melting point, made for audio applications. There is probably a different ratio of tin/lead, than the standard solder. This is standard, for audio soldering.
And then check for shorts with a meter, and when screwing the cover back on, be very careful not to twist the inside wire. I did that the first time, and broke both ground wires. Plugged it in, no sound, checked with a meter and had continuity on the T and R, but no connection on the L- and R-. So open it up, strip more of the wire, solder it, and very carefully screw the cover back on. Worked this time. Should have followed standard practice of doing a final check with a meter after it is fully assembled, but I'm getting lazy and over confident.
Of course, the IEM cable must have separate L and R ground leads at the plug end. Probably all cables do, but if some manufacturer commoned the grds at the Y splitter, and used one Grd lead from the Y to the TRS plug, then of course you cannot split out the L and R Grds onto separate contacts on the TRRS plug, as there is only one grd wire. Just something to be aware of, even though it is very unlikely.
For those without the necessary soldering tools or skills, then you'll need to get a custom cable made up with a TRRS plug. This would be a good option if you are upgrading the cable anyways, so just order the cable with a TRRS plug termination. (Specify for audio, not for microphone.)
The wiring of a TRRS plug for audio is:
T - Tip, L+ Left signal
R - Ring, R+ Right signal. I use the mnemonic of Red=Right to remember which is R and L on a TRS (Tip, Ring, Sleeve) plug, as the Ring lead is always Red, so Ring/Red=Right, and Ring, Red and Right, all start with the letter R, which must be for Right. And that leaves the Tip as being the Left signal. And Sleeve is the remaining contact, which has both L and R Grds connected to it.
R - Ring, L- Grd
S - Sleeve R- Grd
I couldn't paste a picture of a TRRS plug, but a search on the web for TRRS will show it.
Here's a link to one article.
https://robrobinette.com/BalancedCable.htm#Make_a_HiFiMan_HE-500_Balanced_Cable_
!
I have never tried current based amplifiers before and still a little skeptical about it. Did a quick search on google but all the technical hoo-haa doesn't help me understand it either. Hope to be able to demo a set of the QP1R here in Singapore someday 
